Tungsten filament having uniform concentration gradient of thoria particles

ABSTRACT

An improved tungsten filament is disclosed containing approximately 1-2 percent by weight thoria particles distributed in the tungsten matrix as a dispersed phase. The thoria particles are distributed in the tungsten matrix to provide a slight but uniform compositional gradient which diminishes continuously with increasing distance from the exterior surface of the filament to the longitudinal central axis. Upon recrystallization of the filament or wire product there is also formed a tungsten grain structure having a more uniform size and shape of the individual tungsten grains than can be obtained by conventional powder metallurgy preparation.

aet 1 Dunham tates TUNGSTEN FILAMENT HAVING UNIFORM CONCENTRATIONGRADIENT OF THORIA PARTICLES [75] Inventor: Thomas E, Dunham, ClevelandHeights, Ohio [73] Assignee: General Electric Company, New

York, NY.

[ Notice: The portion of the term of this patent subsequent to Mar. 19,1991, has been disclaimed.

[22] Filed: Sept. 1, 1972 [21] Appl. No.: 285,939

[52] US. Cl 29/1825, 75/206, 75/207 [51] Int. Cl. C22c l/05, C220 19/00[58] Field of Search 75/207, 206, 176;

[56] References Cited UNITED STATES PATENTS 12/1913 Coolidge 75/207 X7/1957 lredell et a1 75/207 X 1*Dec. 10, 1974 Dempsey 75/207 X 3,105,29010/1963 Sackinger 75/207 X 3,278,281 10/1966 Ehringer 75/207 X 3,475,15910/1969 Hansen 75/207 X Primary Examiner-Leland A. Sebastian AssistantExaminerR. E. Schafer Attorney, Agent, or Firm-John F. McDevitt; HenryP. Truesdell; Frank L. Neuhauser [S 7] ABSTRACT 6 Claims, 3 DrawingFigures PATENTEB SE81 0 I974 SHEET 10F 3 STHNDHPD 2.0% 7770 MQTEP/RLPRESENT /.8% W70 MQTER/HL PATENTEU DEC] 0 I974 SHEET 2 OF 3 STRNDHPD2.0% 7' I70 MATERIAL PRESENT L8? 7770; METER/9L PATENTEL 51E] DIEM SHEET3 BF 3 5 THNDRRD 1.0% 7/70; F/LflMEN T PRESENT 1.7% 7770 F/LIQMENTTUNGSTEN FILAMENT HAVING UNIFORM CONCENTRATION GRADIENT OF THORIAPARTICLES CROSS-REFERENCE TO RELATED APPLICATIONS Processes for theproduction of binary alloys of the present invention are described inmore detail and are claimed in my copending application entitled METALPRODUCTS AND PROCESS OF PREPARATION," Ser. No. 186,143, filed Oct. 4,1971, now US. Pat. No. 3,741,734. Another copending application filed inmy name which describes and claims related dispersion alloy products isentitled TUNGSTEN ALLOY PRODUCTS, Ser. No. 248,933 and was filed May 1,1972. A further copending application entitled Vibration Resistant Lamp,Ser. No. 285,938 and filed Sept. 1, 1972, discloses and claims anincandescent lamp wherein the filament member utilizes the dispersionalloy material of the present invention. All of these copendingapplications are assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION Thoriated tungsten materials are commonlyused as incandescent filaments in electric lamps and as an electronsource in such items as power tubes, electric discharge lamps andwelding electrodes. It has also been recognized for some time that ahigher thoria level in the tungsten material will produce a higherrecrystallization temperature and greater mechanical strength. It isalso well recognized for thoriated tungsten materials made byconventional powder metallurgy preparation that fabrication of thematerial into a filament or wire product becomes more difficult withincreasing thoria level. As a result of this latter difficulty inprocessing a dispersion alloy product wherein a powdered compact isprepared from a mixture of the solid starting materials or by Th(NO3)4'additions to the tungsten oxides there are two general types of thealloys being manufactured at this time. More particularly, a 2 percentby weight thoria containing alloy is employed for product applicationsnot requiring a highly wrought or small size filament part whereas a 1percent by weight thoria containing alloy is being manufactured forsmaller diameter lamp and electronic tube filament applications.

Although there has been a long period of production for the two generaltype tungstenalloy materials above specified, there remains continuingproblems. The manufacture of 2 percent thoria material to a finishedsize is often accompanied by material efficiencies below 50 percent.While the 1 percent thoria alloy is more fabricable, it can also becharacterized by a nonuniform tungsten grain structure that occurs uponrecrystallization wherein the individual tungsten crystals are unequalin size and develop with a pronounced unevenness in growth along themajor wire axis.

A still further problem exists with both types of binary alloys abovespecified commencing with a dry blend of the starting materials or Th(No h addition to tungsten oxide which is attributable at least in partto non-uniform dispersion of the thoria additive resulting from thedeformation and fracture of the larger ThO particles during manufacture.Since the thoria additive can obstruct grain growth of the individualtungsten crystals during the recrystallization which takes place 2 in awire which has been produced by swaging and wire drawing of an ingot,the above-mentioned non-uniform Th0 dispersion resulting from thedeformation and fracture of the larger Th0 particles will tend to causelarger elongated grains to form in some regions of the wire.

It would be desirable, therefore, to provide a tungsten filamentcontaining more than 1 percent by weight thoria particles which can befabricated readily down to small sizes with a minimum of difficulties.

It would also be useful to provide a tungsten filament containing morethan 1 percent by weight thoria particles with a uniform and small grainsize of tungsten crystals upon recrystallization.

SUMMARY OF THE INVENTION It has now been discovered that binary alloysof tungsten containing approximately 1-2 percent by weight thoriaparticles can be prepared in a particular manner hereinafter more fullydescribed to provide a dispersion alloy product which followingmechanical working to wire recrystallizes to smaller size and moreequiaxed crystals of tungsten than can be obtained by powder metallurgypreparation. More particularly it has been discovered that uponincorporating the above specified thoria levels in a powdered compact oftungsten crystals utilizing a liquid diffusion technique described andclaimed in my copending application Serial No. 186,143 there is obtaineda more narrow size distribution of the thoria particles than can beobtained by preparing the compact starting with a dry blend of thealloying constituents. When such a dispersion product containing thoriaparticles less than about 4 microns in diameter is further processed inthe conventional manner to obtain a rod, filament, or wire product, thespacial distribution of ThO particles remains more uniform than in thecase of conventionally made materials. For example, as will be seen inFIG. 1, most of the large ThO particles in a 93 mil rod W-2 of w/o ThOmade by conventional techniques are rod-like segments strung out alongthe rod axis. The number of this type of particle segment seen in thenew material having approximately the same ThO level is markedly less asa result of the improved ThO distribution at the starting ingot stage ofprocessing. The final tungsten wire produced in accordance with thepresent invention consists of a dispersion strengthened alloy having arelatively small and stable recrystallized tungsten grain size with amore uniform spacial distribution of ThO attributable to the absence ofextremely large ThO particles in the precursor ingot. It is also furtherpossible to distribute the thoria particles in a tungsten compact sothat a uniform thoria concentration gradient exists with a maximumconcentration being located at the exterior surface portion of thecompact and with said thoria concentration continuously diminishing withincreasing distance to the interior portion of said compact. By varyingthe thoria concentration in this manner, it can be further expected thata thoria concentration gradient can be produced in the final wirefilament such that maximum thoria concentration will be located at theexterior surface portion of the filament and the thoria concentrationwill continuously diminish with increasing distance to the longitudinalcentral axis of the filament.

FIG. 1 are photographs taken at 2000 x magnification depictingcomparative ThO distribution for 93 mil diameter rods fabricated from abinary alloy of the present invention compared with a conventional W-2w/o ThO alloy.

FIG. 2 are photographs also taken at 2000 x magnification and depictingthe Th distribution in a sintered compact prepared according to thepresent invention compared with the ThO distribution for a correspondingsintered compact prepared by a conventional powder metallurgy technique.

FIG. 3 are photographs depicting the recrystallized tungsten grainstructures for conventional W-l w/o Th0: material compared with W-l .7w/o ThO alloy of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the liquid diffusionprocess practiced in the present invention comprises soaking a porouscompact of tungsten particles in an inert liquid solvent for a solublethorium compound until the porous structure of the compact isessentially filled with the solvent and thereafter immersing thesolvent-filled compact in a liquid solution of the thorium compound fora sufficient time period to permit liquid diffusion of the dissolvedthorium compound into at least a portion of the solvent contained in thepores of the compact. The compact is then removed from the solutionafter a predetermined amount of liquid diffusion has taken place and allsol- I vent is removed to retain a bulk thoria concentration betweenapproximately 1-2 percent by weight in the pores of the compositeformed. By inert liquid solvent" is meant a solvent for the thoriumcompound which can be removed by drying or heating the liquidfilledcompact without leaving a residue in the pore structure or undergoingany significant chemical reaction with the tungsten material. Inmodifications of the above-described liquid diffusion process, there canbe established a concentration gradient of the thorium compound in thepores of the composite such that a slight but uniform decrease in theresultant thoria concentration can be established in the direction ofthe center.

A specific example will now be given to illustrate preparation of animproved tungsten filament according to the present invention.Accordingly, a pressed compact weighing approximately 3.0 kilograms andhaving a rectangular cross section of 17 mm X 22 mm with a density of10.9 gms/cc was prepared from tungsten powder having an average particlesize of 2.5 microns by conventional techniques. The compact was slowlysoaked in distilled water until the interconnected pore structure of thecompact was saturated and the liquid-filled ingot thereafter immersed inan aqueous Th(NO solution having a concentration of 520 grams Th0:per'liter of solution. The liquid diffusion process was interruptedafter approximately 30 hours by removing the compact from the thoriumnitrate solution. The compact was next dried slowly in a vacuum oven atapproximately l30F which removed most of the solvent from the pores ofthe compact while retaining a slight concentration gradient of thethorium additive in the pores at a bulk ThO concentration of 1.9 weightpercent. The thorium additive-containing compact was next subjected to aconventional presintering operation at approximately 1200C in hydrogenfor two hours which converted the thorium nitrate in the pores tothorium oxide (thoria).

The presintered compact of the present example was then sintered inconventional fashion by passing an electrical current through the memberto raise its temperature nearly to the melting point. The sinteringtreatment was provided by suspending the presintered compact verticallybetween electrodes and applying electric current in graduated steps toprovide for an escape of any volatile impurities in the compact.

Test specimens were obtained from the sintered compact which had arectangular cross section in order to measure the thoria concentrationalong with the distribution and size-range of the thoria particles. Thetest specimens were examined by known autoradiograph and extractionreplica techniques. In one cross-section specimen, the thoriaconcentration measured by standard X-ray techniques was approximately2.1 weight thoria at the corner of the compact with the concentrationdecreasing to approximately 1.9 weight percent thoria at the centerlongitudinal axis of the compact. The character of the thoria particlesize in the present compact as observedv by examining extractionreplicas in the electron microscope was also found to be significantlydifferent than that obtained with conventional powder metallurgypreparation as shown in FIG. 2. First of all, the thoria particle sizerange in the present material was significantly smaller than in theconventional material of the same bulk composition. Secondly, there wasnot found to be any significant agglomeration or segregation of thoriaparticles in the present material and the size of the larger thoriaparticles in the present material was significantly smaller than wasfound in the conventional material. No thoria particles having aparticle size larger than approximately 4 microns diameter was observedin the examined specimen whereas it is not uncommon to see ThO particlesin excess of 7 microns in conventional W materials containing 2% ThOFollowing the sintering step, the compact was rolled, swaged and drawnto approximately 18-mil wire in accordance with further conventionaltungsten working practices. The final tungsten wire was furthersubjected to recrystallization by passing an electrical current throughthe wire in a hydrogen atmosphere. More particularly, the wire wassubjected for a 6-minute time period, to approximately percent of itsfusion amperage which produced a smaller and more uniform tungsten grainsize than is conventionally obtained for either 1 w/o or 2 w/o ThOalloys. The more uniform recrystallized grain structure is a result of asmaller number of ThO particle segments aligned parallel to the wireaxis in the new material.

A further comparison of the recrystallization structure was made betweentungsten filaments produced in accordance with the present invention andthe same diameter filaments produced in conventional fashion butcontaining only 1% by weight of thoria. When a 7-mil diameterconventional product was recrystallized by heating for 5 minutes at2,650C, there was obtained a recrystallization structure characterizedby some large elongated grains.As distinct therefrom, therecrystallization structure of a wire product having the same diameterbut prepared in accordance with the present invention and containingapproximately 1.7 percent thoria by weight after heating for 6 hours at2,650C lacked the large grain size and non-uniformity in grain structureof the conventional product. The comparative grain structures aredepicted in FIG. 3. The same distinctions are even more pronounced atthe 2.2-mil significantly higher than for the 1 percent thoria alloyprepared conventionally while the ductility as measured by percentelongation is also equal to that of the 1 percent material. Thissignificant increase in ductility 5 was not expected for the new W-l.7%w/o ThO matewire size which provides confirmation of the more unirial.form size and shape of the recrystallized tungsten It will be apparentfrom the foregoing description grains that are obtained in accordancewith the present that various modifications can be employed to obtaininvention. Samples ofa recrystallized tungsten filament the products ofthe present invention. For example. having such improved grain structurehave also been 10 various modifications can be made in the liquiddiffuobserved to exhibit higher ductility than is obtained by siontechnique with comparable results. Likewise. difconventionalpreparation. The preferred bulk concenferent processing steps can beemployed than above detration of thoria in a tungsten compact to producescribed in fabricating final tungsten filament from the these desirableresults is between 1.3 and 1.9 weight impregnated compact withoutdeparting from the true percent spirit and scope of the presentinvention. It is intended As a still more definite indication of therelationship to limit the present invention, therefore, only to thebetween tensile strength and ductility for alloy materiscope f thfollowing claims. als of the present invention, certain further physicalWh t I laim a new and de ire to secure by Letters tests are beingreported. More particularly, actual P t f h United States is;measurements of the yield strength, ultimate tensile 1, A th lly andmechanically worked dispersion Strength and percent elongation were madep alloy member consisting essentially of a recrystallized mil diameterspecimens from two d1fferent1ngots of the tungsten grain Structure h rin the tungsten grain yP of thoriated tungsten all0y Product describedare small size equiaxed crystals and which further conthe above speclficP The Same P y meastains an average l-2 percent by weight thoriaparticles uremems were made p 18-ml] dlameter Speclmens of smaller sizethan the tungsten grains distributed in a Obtained from thorlatedtungsten alloyproducts. uniformly diminishing concentration gradienthaving tammg 1 Percent and Pirrierlt y welght thoria but the maximumconcentration located at the exterior sur- Prepared by a ConvemlonalPowder metallurgy l face of said member with no sharp compositionalvariaq All test Specrmens were sublected to a mmute tion of thoriaparticles along said concentration gradianneal in vacuum at 2,400C,prior to the physical tests which were conducted at temperatures fromroom tem- 2 A swaged member having the alloy composition perature to2,500C. The strain rate during these physid fi d in claim 1. Cal testswas 005 Inches P mch P mmut? A 3. A rolled member having the alloycomposition deparison of tests results for the various specimens apfi din claim 1 Pears in Table I below- A drawn filament having the alloycomposition de- TABLE I YIELD STRENGTH (PSI) ULTIMATE STRENGTH (PSI)ELONGATION Present Present Present Alloy Alloy Alloy Temp 2% 1% 1.7% 2%1% 1.7% 2% 1% (1.7% (C) T110 Tho ThO Tho Tho ThO Tho Tho ThO It can benoted in Table I that tensile strength for the thoriated tungsten alloyproduct increases with an increasing level of thoria concentration. Theabove reported ductility measurements for the conventionally prepared 1percent and 2 percent by weight thoria alloys also generally indicatethat ductility is reduced with increasing thoria levels. In contrast tothis latter finding, however, the yield and ultimate strengths for the1.7 percent thoria alloy of the present invention are fined in claim 1.

5. A dispersion alloy member as in claim 1 having improved strength andductility compared with a conventionally prepared dispersion alloymember of the same composition.

6. A drawn filament having the physical characteristics defined in claim5.

1. A THERMALLY AND MECHANICALLY WORKED DISPERSION ALLOY MEMBERCONSISTING ESSENTIALLY OF A RECRYSTALLIZED TUNGSTEN GRAIN STRUCTUREWHEREIN THE TUNGSTEN GRAINS ARE SMALL SIZE EQUIAXED CRYSTALS AND WHICHFURTHER CONTAINS AN AVERAGE 1-2 PERCENT BY WEIGHT THORIA PARTICLES OFSMALLER SIZE THAN THE TUNGSTEN GRAINS DISTRIBUTED IN A UNIFORMLYDIMINISHING CONCENTRATION GRADIENT HAVING THE MAXIMUM CONCENTRATIONLOCATED AT THE EXTERIOR SURFACE OF SAID MEMBER WITH NO SHARPCOMPOSITIONAL VARIATION OF THORIA PARTICLES ALONG SAID CONCENTRATIONGRADIENT.
 2. A swaged member having the alloy composition defined inclaim
 3. A rolled member having the alloy composition defined in claim4. A drawn filament having the alloy composition defined in claim
 1. 5.A dispersion alloy member as in claim 1 having improved strength andductility compared with a conventionally prepared dispersion alloymember of the same composition.
 6. A drawn filament having the physicalcharacteristics defined in claim 5.